Interest in metallocene and non-metallocene single-site catalysts (hereinafter all referred to as single-site catalysts) continues to grow rapidly in the polyolefin industry. These catalysts are more reactive than Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of .alpha.-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
Recent attention has focused on developing improved single-site catalysts in which a cyclopentadienyl ring ligand of the metallocene is replaced by a heteroatomic ring ligand. For example, U.S. Pat. No. 5,554,775 discloses catalysts containing a boraaryl moiety such as boranaphthalene or boraphenanthrene. U.S. Pat. No. 5,539,124 discloses catalysts containing a pyrrolyl ring, i.e., an "azametallocene." In addition, PCT Int. Appl. WO 96/34021 discloses azaborolinyl heterometallocenes wherein at least one aromatic ring includes both a boron atom and a nitrogen atom.
Single-site catalysts that contain heteroatomic ring ligands ("heterometallocenes") are often quite challenging to synthesize. There continues to be a need for heterometallocenes that can be prepared inexpensively and in short order.
U.S. Pat. No. 5,637,660 discloses single-site catalysts that contain a Group 4 transition metal (such as titanium or zirconium) and at least one quinolinyl or pyridinyl group. When combined with an activator such as MAO or an ionic borate, these catalysts efficiently polymerize olefins such as ethylene or mixtures of ethylene and .alpha.-olefins. The ready availability of quinolinols and pyridinols and ease of preparation make these catalysts an attractive alternative to other heterometallocenes.
The '660 patent illustrates a relatively simple, two-step method for making the catalysts (see Example 3). First, the quinolinol or pyridinol reacts with an equivalent of butyllithium in toluene to give a slurry of the lithium quinolinolate or pyridinolate salt. The lithium salt is then combined with one molar equivalent of a Group 4 transition metal tetrahalide (e.g., titanium tetrachloride) to give the desired quinolinoxy or pyridinoxy-substituted transition metal trihalide. For example, the reaction of lithium 8-quinolinolate with an equimolar amount of titanium tetrachloride gives 8-quinolinoxytitanium trichloride. Unfortunately, the isolated yield of the desired transition metal catalyst is only 20-25%. Better yields are clearly needed.
Frazer et al. (J. Chem. Soc. (A) (1966) 544) disclose reactions of 8-quinolinol with covalent halides such as titanium or tin tetrahalides. Frazer found that "adducts" (rather than reaction products) are obtained when titanium tetrahalide is combined with one or two equivalents of 8-quinolinol at room temperature in chloroform. When two or more equivalents of 8-quinolinol are used, Frazer isolates a reaction product, which is primarily bis(8-quinolinoxy)titanium dichloride (see reaction scheme at column 1, page 545 of the reference). 8-Quinolinoxytitanium trichloride is made from the bis(8-quinolinoxy) compound by reacting the latter with an equimolar amount of titanium tetrachloride in chloroform.
In sum, an improved way to make single-site, heterometallocenes is needed. In particular, better ways to make heterometallocenes from readily available quinolinols and pyridinols would be valuable. Ideally, the method would be easy to practice and would give high yields of olefin polymerization catalyst precursors.